Story highlights

Clinton delivered a blistering assessment of Bernie Sanders' credentials and readiness to be President

Clinton's campaign also released a video where a senior policy aide argues the Vermont senator "just hasn't thought it through" on Iran

Part of the speech will be used in an ad as well

Indianola, Iowa (CNN)Hillary Clinton delivered a blistering assessment of Bernie Sanders' credentials here on Thursday and implored Iowa voters to scrutinize his policies and readiness for the White House, declaring, "Theory isn't enough. A President has to deliver in reality."

It was the most forceful and direct contrast Clinton has drawn with Sanders yet, a speech that underscored the increasing urgency and acrimony of the race. From health care to foreign policy, Clinton repeatedly referenced Sanders by name and questioned whether his ideas could ever become reality.

"I am not interested in ideas that sound good on paper but will never make it in the real world," Clinton said. "I care about making a real difference in your life and that gets to the choice you have to make in this caucus."

Clinton acknowledged that while she and Sanders "share many of the same goals" they have "different records and different ideas on how to drive progress."

The former secretary of state used a Teleprompter to deliver her remarks to hundreds of supporters on the campus of Simpson College. The speech, one adviser said, was designed to "shake some sense into Iowans" and escalate the experience argument she has been making against Sanders with limited success.

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"Senator Sanders doesn't talk much about foreign policy, but when he does it raises concerns," Clinton said. "Sometimes it can sound like he really hasn't thought it through."

Clinton's campaign had multiple cameras here and plan to turn part of the speech into an ad, according to aides.

Attacks on foreign policy

Clinton hammered Sanders as someone who "hasn't thought it through" on a number of foreign policy issues Thursday, including Iran.

"He has suggested that we invite Iranian troops into Syria," Clinton said. "That is like asking the arsonist to be the firefighter. ...The challenges a President has to grapple with are beyond complicated but at home and abroad. That is why is it is the hardest job."

The former secretary of state said she is the "only candidate on either side with the experience and judgment to keep us safe at home and strong in the world."

Sanders' campaign has responded to the critiques by questioning Clinton's judgment and noting her 2002 vote for the Iraq War, something that then-Sen. Barack Obama used against her during her failed 2008 campaign.

Importance of Iowa

The uptick in rhetoric comes in the final days of the Iowa campaign. Democrats will choose who they want as their nominee on February 1 and both external and internal polls show the race is close.

Iowa's importance has only increased in recent days, given a string of polls that show Sanders leading in New Hampshire.

The former first lady also blasted Sanders plan for single-payer healthcare, casting it as something that would only give Republicans "an opening to come in and tear down everything we have achieved" with the Affordable Care Act.

"I know Senator Sanders cares about covering more people, as I do, but rather build on the progress that has been made, he wants to start over from scratch with a whole new system," she said.

Fire against Sanders was not the only theme of Clinton's speech. The former senator also hit Republicans, as she does regularly, and showed a more contemplative side, something she displayed Wednesday in Burlington, Iowa.

"I can't do any of this alone. I need you," Clinton said Thursday during a speech where she acknowledged that she knows "what it is like to be knocked down."

"I have had a few hard times in my own life and I doubt there is anybody who hasn't had their share," Clinton said. "But I learned from my mom and my faith that it is not whether you get knocked down it is whether you get back up."

Video against Sanders on Iran

Earlier Thursday, Clinton's campaign released a video where a senior policy aide argues the Vermont senator is unprepared on Iran.

Jake Sullivan, Clinton's senior policy adviser and a lead negotiator on the Iran deal, challenges Sanders' call for normalize relations with Iran.

"Secretary Clinton supports diplomacy with Iran. In fact, she helped launch the diplomacy that helped," Sullivan says. "But normal relations with Iran right now? President Obama doesn't support that idea. Secretary Clinton doesn't support that idea. And it's not at all clear why it is that Sen. Sanders is suggesting it."

After listing some of Sanders' foreign policy ideas, Sullivan said, "When you look at all of these ideas, it's pretty clear that he just hasn't thought it through."

At Sunday's debate, Sanders said it was time for the United States to "move as aggressively as we can to normalize relations with Iran, understanding that Iran's behavior in so many ways in something that we disagree with."

"Their support for terrorism, the anti-American rhetoric that we're hearing from some of their leadership is something that is unacceptable," Sanders said. "On the other hand, the fact that we managed to reach an agreement, something that I very strongly supported, that prevents Iran from getting a nuclear weapon and that we did that without going to war and that I believe we're seeing a thaw in our relations with Iran is a very positive step."

And on Tuesday, Clinton's campaign released a joint statement from 10 former Obama and Clinton administration diplomats who are questioning Bernie Sanders' ability to handle foreign policy issues, including working with Iran and fighting ISIS.